The present invention relates to shock and vibration isolators used to support high precision or delicate equipment. Many precision optical instruments such as, for example, photograph mask aligners used in the production of semiconductor devices are adversely affected by shocks and vibrations transmitted through the floor or the work surface on which they sit.
Various types of vibration damping devices, such as rubber pads or feet are helpful, but some instrument systems require greater isolation than can be provided by such passive devices. Gas filled shock absorbers are frequently used for instrument systems that are heavy and require high isolation. Prior art gas-filled shock absorbers were usually one of two types. One type was essentially a gas-filled rubber bag with a mechanically controlled valve. The valve was coupled to the supported device so that gas would be added to or exhausted from the bag in response to relative movement between ground and the supported device. One of the major disadvantages of gas-filled rubber bags is their large size. Another disadvantage is the limited feedback characteristics of the mechanically controlled valve. It is sometimes difficult to make a system with these valves stable and it is difficult to compensate such a system to improve its stability. In addition, a gas-filled air bag does not work well with utilization devices that are relatively tall and narrow and thus have a relatively high center of gravity.
A second type of prior art shock absorber is an air piston that is constructed using a commercially available sealing device known as a rolling Bellowfram. This piston fits loosely in an air cylinder and the piston to cylinder seal is effected by the Bellowfram which is essentially a flexible diaphragm. The piston can move along the vertical axis by the additon or removal of gas, as with the rubber bag. Because of the rolling action of the Bellowfram and the compressibility of the gas, this device has a relatively high compliance along the vertical axis to isolate against shocks along that axis. However, the compliance of this device is lower in the horizontal plane than along the vertical axis whereas the rubber bag can be made to have equal compliance along all three axes. Thus, the air piston using a Bellowfram does not isolate against shocks in the horizontal plane as well as the rubber bags do. Air pistons using Bellowframs were also controlled by a mechanically actuated valve and thus have had the response and stability disadvantages that were described above.